Stereo speaker systems have been used in numerous audio applications. A stereo speaker system usually generates a sound stage that is restricted by the physical locations of the speakers. Thus, a listener would perceive sound events limited to within the span of the two speakers. Such a limitation greatly impairs the perceived sound stage in small-size stereo speaker systems, such as those found in portable devices. In the worst cases, the stereo sound almost diminishes into mono sound.
To overcome the size limitation of small stereo systems and widen the sound stage for general stereo systems, 3D sound generation techniques may be implemented. These techniques usually expand the stereo sound stage by achieving better crosstalk cancellation, as well as enhancing certain spatial cues. However, the 3D effects generated by a stereo speaker system using conventional 3D sound generation techniques are generally not satisfactory because the degrees of freedom in the design are limited by the number of speakers.